Upward Trends in Nuclear Technology
Nuclear reactors have been used for the past 6 decades as a leading source of low-carbon power generation, producing around one gigawatt of power without producing a recordable amount of greenhouse gases. In terms of total life-cycle greenhouse gas emissions per unit of energy generated, it has emission values comparable to or lower than renewable energy.
However, nuclear reactors have their challenges. Due to licensing requirements for their custom designs, nuclear reactors can take more than a decade to be approved and built. Public safety concerns also exist due to high visibility incidents that have incurred in the past, despite a relatively good track record of safety. Further, nuclear power has only been implemented terrestrially likely due to reasons cited above coupled with the challenges of operating in a space environment.
Cue the Mini Reactors
To combat the issues with custom design and construction, the concept of Small Modular Reactors (SMR) was introduced, which can produce up to 300MW of power. SMRs are manufactured in a facility to set specifications and then shipped to the site, simplifying some regulatory hurdles. Since each reactor is identical, once one is approved, additional reactors can easily be ordered, manufactured, and installed as demand grows.
Microreactors are even smaller modular designs that produce up to 10MW of power. Their first intended use is for remote locations, including military bases, and locations in Canada and Alaska. Due to transportation costs, diesel fuel for generators can cost 10 times more than normal when shipped to these remote villages, as well as military bases. The microreactors will provide clean, efficient, appropriately sized power sources with minimal environmental impact.
A key feature of many microreactors is the high-temperature heat pipes, designed to make use of the phase change of the working fluid to transport a large amount of heat from the reactor to the power conversion devices with very small temperature drops. Due to their unique construction, high-temperature heat pipes can be used to build custom heat transfer devices for both high-power throughput and low ΔT.
In all nuclear reactors, heat from the reactor is transferred to a series of power converters to generate energy. The waste heat from the conversion process is removed and rejected. Most standard reactors and SMRs have used high-temperature and high-pressure water to both supply and remove heat from the power converters.
With microreactors, the power to the converters can be supplied with passive alkali-metal heat pipe, where the evaporator end of the heat pipe is inserted into the core, and the condenser end supplies heat to the converters. This eliminates the need for a pump, increasing reliability. The heat pipes also transfer heat at a higher temperature, increasing the thermodynamic efficiency. ACT is currently working with several customers to incorporate heat pipes in their designs.
Nuclear Power… in Space?
NASA has expressed an interest in space nuclear power in a variety of applications ranging from habitat power to deep space missions to nuclear propulsion.
Nuclear thermal propulsion and Nuclear Electrical Propulsion have both been considered for future manned missions to Mars. Compared to traditional chemical propulsion systems, nuclear propulsion is more reliable, providing high thrust and double the propellant efficiency. NASA is looking to use this technology for crewed missions to Mars, as it allows for more flexible abort scenarios, including immediate departure from Mars if necessary. Nuclear propulsion is in the 10-100MW range, similar in size to a Small Modular Reactor or SMR.
Kilopower Project Leads to NASA’s Fission Surface Power
NASA has also been studying powering lunar and Martian habitats. Power expectations for these deployments are on the order of 10-50kW, similar to a microreactor. ACT has developed alkali metal heat pipes and thermosyphons for transferring heat from the micro-reactor core to the power converters in NASA’s Kilopower program. Due to the lack of atmosphere, the waste heat from the converters must be rejected by a radiator.
Since there is no atmosphere in space, the waste heat from the convertors must be rejected by radiation. ACT has been working on developing heat pipe radiators for nuclear systems in space since its founding. Typical spacecraft heat pipes use aluminum with ammonia as the working fluid and operate below 40°C. Fission radiators are normally designed to operate around 250°C, so ACT developed titanium/water gas-charged heat pipes (a form of Variable Conductance Heat Pipes (VCHPS)) and demonstrated their compatibility. The use of VCHPs means that the reactor power can be turned down and restarted, improving efficiency. The radiators have Graphite Fiber Reinforced Composite (GFRC) Facesheets, accommodating the C.T.E. mismatch between the GFRC and the titanium. ACT currently has a non-integrated, hot-reservoir VCHP on-board Astrobotic’s Peregrine Lander which is set to launch in 2023.
Small Scale Nuclear, Big Time Innovation
Innovation for Small Modular Reactors or SMRs and microreactors has been in development for many years. With government incentive programs pushing for carbon emissions reduction to lower greenhouse gases, the idea of the small modular reactor is a tangible and highly anticipated reality. With continued research and development, cleaner, more efficient power will be more easily accessible and readily available no matter where you are located geographically. The function of high-temperature heat pipes will continue to aid in the efficiency and overall carbon neutrality of the nuclear industry, here on earth and beyond.
ACT has recently completed a NASA Phase I program on “Hot End Thermal Management System for Nuclear Electric Propulsion”. Nuclear electric propulsion systems allow for increased science payload, reduced flight times and longer mission lifetimes. The thermal management system linking the reactor to the hot end of the power conversion system must be efficient, lightweight and reliable while operating at the MW scale. The Phase I program demonstrated high-power heat pipes carrying 3kW over a meter, a full-scale 10MW reactor design, and a heat pipe based thermal management system capable of high heat flux (>0.3MW/m2) and minimal temperature drop (<50K).
Explore ACT’s Impact on Cooling in the Nuclear Industry